1. Field of the Invention
The present invention relates to a solid state image sensor device provided with a microlens which condenses the incident light, and, in particular, to the structure of this microlens.
2. Description of the Prior Art
In a conventional solid state image sensor device 1, as shown in FIG. 1, the incident light normally passes through a camera lens 6 and is focused, after which this incident light is sensed by a picture element 7 having a photosensitive member (not shown), built into the solid state image sensor device 1.
An electric charge produced by photoelectric conversion in the photosensitive member is then transmitted through a vertical charge coupled device (vertical CCD) 8 and a horizontal CCD 9 and discharged from the solid state image sensor device 1 to an external device in the form of a video signal.
Much consideration has been given to methods of improving the sensitivity of the solid state image sensor device 1 such as, for example, by providing a microlens on the surface of the solid state image sensor device 1 which receives the incident light.
FIG. 3 illustrates a solid state image sensor device which is formed on a semiconductor substrate using a structure which provides a microlens for condensing the incident light. In FIG. 3, the solid state image sensor device 1 comprises a photosensitive member 2, for example, a photodiode, which photoelectrically converts the incident light directed onto a surface of the solid state image sensor device 1 to produce an electric charge, and a transmission member, for example, a vertical CCD 8, which transmits this electric charge to an external device. In addition, a transparent flat layer 4 is formed in a level plane on the surface of the solid state image sensor device 1 above the photosensitive members 2 and the vertical CCDs 8. A plurality of microlenses 5 is provided, patterned in a hemispherical shape to correspond to the respective photosensitive members 2 or patterned in a semicolumnar shape, specifically, a striped shape, to correspond to one row of the photosensitive members 2. The microlenses 5 are formed by melting, solidifying and annealing a lens material.
A solid state image sensor device with this type of structure is described in the literature (Y. Ishihara et al.; IEDM 83, p. 497 (1983)). With such a device it is possible to improve the sensitivity to about double that of the case where the microlenses are not formed.
In a solid state image sensor device with this type of structure, used, for example, in a camera such as a video camera, the sensitivity or the smear characteristics of the solid state image sensor device are improved when the incident light passes through a camera lens. Specifically, these characteristics change according to the change in the F value of the camera lens (F=f/D, where F is the brightness, f is the focal distance, and D is the aperture of the camera lens).
The relationship between the relative sensitivity of the solid state image sensor device 1 and the F value of the camera lens 6 is shown in FIG. 4. As can be clearly understood from FIG. 4, which shows the case where the film thickness of the microlens 5 changes between 2.3 .mu.m and 2.8 .mu.m, as the F value of the camera lens 6 decreases, the relative sensitivity of the solid state image sensor device 1 also decreases.
As shown in FIG. 5, the change in the F value is based on the relation between the aperture D and the angle of vision .theta. of the camera lens 6. For example, as the F value of the camera lens 6 decreases, the angle of vision .theta. increases, which means that the incident light enters obliquely. When F=1.4, for example, the angle of vision .theta.=19.7.degree., and when F=11 the angle of vision .theta.=2.6.degree.. As a result, when the angle of vision increases in accordance with a decrease in the F value, as shown in FIG. 6, compared to the case where the F value is large the optical path of the incident light changes, and the incident light has difficulty in entering the photosensitive member 2 of the solid state image sensor device 1.
Accordingly, the amount of light provided to the photosensitive member 2 decreases and the sensitivity also decreases. However, in a camera using the solid state image sensor device 1, such as a video camera, not only is a size reduction required, but a small F value and a high sensitivity must also be provided. As outlined in the foregoing explanation, when a video camera or the like is incorporated in a conventional solid state image sensor device 1, a reduction in the F value of the camera lens 5 is accompanied by a decrease in the relative sensitivity.
For this reason, when the size of the camera lens 5 of a video camera or the like is reduced, a solid state image sensor device with a small F value and a high sensitivity is required. However, it is impossible to meet these requirements.